Optical pickup device and method for designing the same

ABSTRACT

An optical pickup device is equipped with a first light source that emits first light, a second light source that emits second light having a wavelength different from a wavelength of the first light, and a diffraction element that deflects the first light or the second light to match optical axes of the lights. The diffraction element is a step-like diffraction element in which one of an incident face and an emitting face thereof has a step-like grating face. A step difference of the step-like grating face is set to have a measurement that generates a phase difference of one wavelength of one of the first light and the second light, and the number of steps of the step-like grating face is set to maximize a (+) first order diffraction efficiency or a (−) first order diffraction efficiency for the other light.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a two-wavelength optical pickupdevice that is used for recording and reproducing data on opticalrecording media such as CDs (compact discs), DVDs (digital versatilediscs) and the like. More particularly, the present invention relates toa step-like diffraction element that is used for matching optical axesof two lights having different wavelengths that are emitted from lightsources located at different emission points in the two-wavelengthoptical pickup device, and a method for designing such a step-likediffraction element.

[0003] 2. Description of Related Art

[0004] CDs and DVDs, which are mutually different in their substratethickness and recording density, are known as optical recording media.When recording or reproduction of data is conducted with respect tothose optical recording media, laser beams of different wavelengths maybe required. For example, while a laser beam with a wavelength of 650 nmis required to reproduce data on a DVD, a laser beam with a wavelengthof 780 nm is required to reproduce and record data on a CD-R.

[0005] A so-called two-wavelength optical pickup device is known as anoptical pickup device that reads DVDs and reads and records on CD-Rs.The two-wavelength optical pickup device has a laser beam light sourcefor emitting a laser beam with a wavelength of 650 nm and a laser beamlight source for emitting a laser beam with a wavelength of 780 nmmounted as a single light source.

[0006] For example, a conventional two-wavelength optical pickup deviceuses a common optical system for different laser beams in order to makethe device smaller and more compact. For this purpose, one of the laserbeams that are emitted from laser beam light sources at differentemission points is deflected by a diffraction grating to thereby conductboth of the laser beams into a common optical path, and converge them onan optical recording medium through a common objective lens.

[0007] The diffraction grating, which is used as a phase diffractionelement, is formed from a transparent substrate in which one of itsopposing incident face and emission face has a grating surface providedwith gratings having protrusions and grooves. The depth of the gratingis set such that the diffraction grating phase is 2π, in other words, isset at a light-path difference corresponding to one wavelength, for thelaser beam from the laser beam light source with a shorter wavelength.Accordingly, the laser beam from the laser beam light source with theshorter wavelength travels straight path without being affected by thediffraction action, and the laser beam from the laser beam light sourcewith the longer wavelength receives the diffraction action, and itsfirst-order diffraction light obtained by the diffraction is introducedin the common optical path.

[0008] When the wavelength of the laser beam light source with theshorter wavelength that does not receive the diffraction action is 650nm, and the wavelength of the laser beam light source with the longerwavelength that receives the diffraction action is 780 nm, and when therefractive index of the grating material is N and the depth of thegrating groove is d, the diffraction grating is composed in a manner tosatisfy N×d=650 nm.

[0009] In the mean time, a phase difference with respect to the beam ofa wavelength 780 is given by the following expression (1).

N×d/780×2×π=0.833×2π  (1)

[0010] Also the diffraction efficiency of (±) first order diffractionlight with a wavelength of 780 nm is given by the following expression(2).

(2/π)²×sin²(0.833 2π/2)=0.10  (2)

[0011] It is understood from the above that the conventionaltwo-wavelength optical pickup device has a small utilization efficiencyof 0.1 for the laser beam with a wavelength of 780 nm that is diffractedby the diffraction grating, in other words, by a phase diffractionelement. Therefore, in order to perform recording on CD-Rs, a laser beamlight source that is capable of emitting a light amount ten timesgreater than the ordinary amount required. In order to prevent the lightdiffraction efficiency from lowering, a step-like diffraction element inwhich its grating is configured in a step-like manner needs to be usedas a phase diffraction element.

[0012] Also, when the light amount is modulated, the wavelength of asemiconductor laser that is used as a light source varies by several nmdepending on the light amount. As a result, the diffraction anglechanges due to the changes in the wavelength, which causes a deviationin the optical axis. A deviation angle Δθ of the optical axis is givenby the following formula (3).

Δθ=sin⁻¹(Δλ/P)  (3)

[0013] where, Δλ is an amount of the change in the wavelength at thetime of reproducing and recording data, and P is a grating interval.

[0014] Occurrence of deviations in the optical axis at the time ofrecording is not appreciated because this may cause problems such astracking deviations or deformations in pit configurations.

[0015] On the other hand, the optical system that uses two wavelengthssuffers chromatic aberrations in which, for example, focal distances ofthe objective lens and the collimator lens become different depending onthe wavelengths. For this reason, the position of light advancingdirection of a light emission point of each light source must be decidedaccording to the chromatic aberration of the collimator lens. Therefore,the design of a two-wavelength light source is dependent on the designof a collimator lens and therefore hardly has any degree of freedom.

SUMMARY OF THE INVENTION

[0016] In view of the problems described above, it is an object of thepresent invention to provide an optical pickup device equipped with astep-like diffraction element that can conduct lights with differentwavelengths emitted from multiple light sources into a common opticalsystem with good diffraction efficiency.

[0017] An optical pickup device in accordance with on embodiment of thepresent invention comprises a first light source that emits first light,a second light source that emits second light having a wavelengthdifferent from a wavelength of the first light, and a diffractionelement that deflects the first light or the second light to matchoptical axes of the first and second lights. The diffraction element isa step-like diffraction element in which one of an incident face and anemitting face thereof has a step-like grating face, and a stepdifference of the step-like grating face is set to have a measurementthat generates a phase difference corresponding to one wavelength of oneof the first light and the second light, and the number of steps of thestep-like grating face is set such that the number of steps maximizes a(+) first order diffraction efficiency or a (−) first order diffractionefficiency for the other light.

[0018] When the wavelength λ1 of the first light is longer than thewavelength λ2 of the second light, in other words, when the first lightis on the longer wavelength side, and the second light is on the shorterwavelength side, and the step difference of the step-like grating faceis set to have a measurement that generates a phase difference of onewavelength for the first light, the first light on the longer wavelengthside travels straight path through the diffraction element withoutreceiving a diffraction action by the step-like diffraction element.

[0019] In this case, when the number of steps of the step-like gratingface of the step-like diffraction element may be set to be an integerthat is closest to a value α that satisfies an expression,λ2/λ1=α/(α+1), the diffraction efficiency of the (−) first orderdiffraction light of the second light on the shorter wavelength side isimproved. Accordingly, the (−) first order diffraction light may be usedas a light to be emitted from the second light source.

[0020] Conversely, when the step difference of the step-like gratingface of the step-like diffraction element is set to have a measurementthat generates a phase difference corresponding to one wavelength forthe second light, the second light on the shorter wavelength sidetravels straight path through the diffraction element without receivinga diffraction action by the step-like diffraction element.

[0021] In this case, when the number of steps of the step-like gratingface may be set at a value in which one “1” is added to an integer thatis closest to a value α that satisfies λ2/λ1=α/(α+1), the diffractionefficiency of the (+) first order diffraction light on the longerwavelength side is improved. Accordingly, the (+) first orderdiffraction light may be used as a light to be emitted from the firstlight source.

[0022] Other objects, features and advantages of the invention willbecome apparent from the following detailed description taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 schematically shows a structure of an optical system of anoptical pickup device in accordance with a first embodiment of thepresent embodiment.

[0024]FIG. 2 is a cross-sectional view of a step-like diffractionelement to be used in the optical pickup device 1 shown in FIG. 1.

[0025]FIG. 3 is a graph showing relations between grating depths of thestep-like diffraction element shown in FIG. 2 and diffractionefficiency.

[0026]FIG. 4 is a plan view of gratings of the step-like diffractionelement shown in FIG. 2.

[0027]FIG. 5 schematically shows a structure of an optical system of anoptical pickup device in accordance with a second embodiment of thepresent invention.

[0028]FIG. 6 is a cross-sectional view of a step-like diffractionelement to be used in the optical pickup device shown in FIG. 5.

[0029]FIG. 7 is a graph showing relations between grating depths of thestep-like diffraction element shown in FIG. 6 and diffractionefficiency.

[0030]FIG. 8 is a cross-sectional view of a step-like diffractionelement to be used in the optical pickup device shown in FIG. 1.

[0031]FIG. 9 is a graph showing relations between grating depths of thestep-like diffraction element shown in FIG. 8 and the diffractionefficiency.

[0032]FIG. 10 is a cross-sectional view of a step-like diffractionelement of another example to be used in the optical pickup device shownin FIG. 5.

PREFERRED EMBODIMENTS OF THE INVENTION:

[0033] Optical pickup devices in accordance with various embodiments ofthe present invention are described below with reference to theaccompanying drawings.

[0034] (First Embodiment)

[0035]FIG. 1 schematically shows a structure of an optical system of anoptical pickup device 1 in accordance with a first embodiment of thepresent invention. The optical pickup device 1 of the present embodimentreproduces and records data on optical recording media 6 of differenttypes that are different in the substrate thickness and the recordingdensity, such as CDs, CD-Rs and DVDs. The optical pickup device 1 isequipped with a two-wavelength light source (light source unit) 10 thatmay include as a single common package a first laser beam light source11 that emits a first laser beam L1 with a wavelength of 785 nm and asecond laser beam light source 12 that emits a second laser beam L2 witha wavelength of 650 nm, and a common optical system Lo. Light emittingpositions of the first laser beam L1 and the second laser beam L2 areseparated from one another by a specified distance.

[0036] The common optical system Lo includes a step-like diffractionelement 20 that straightly advances the first laser beam L1 emitted fromthe two-wavelength light source 10 and deflects the second laser beam L2emitted from the two-wavelength light source 10 to match optical axes ofboth of the beams, a plate-shaped beam splitter 30 that partiallyreflects the laser beams L1 and L2 whose optical axes coincide with oneanother, a collimator lens 40 that forms parallel light of the laserbeams L1 and L2 that has been reflected by the beam splitter 30, and anobjective lens 41 that converges the laser beams L1 and L2 emitted fromthe collimator lens 40 on a recording surface 6 a of the opticalrecording medium 6. The step-like diffraction element 20 will bedescribed below in greater detail.

[0037] Also, a common light-receiving element 13 is disposed in thecommon optical system Lo to receive returning lights of the first laserbeam L1 and the second laser beam L2 that pass the beam splitter 30after having been reflected by the recording face 6 a of the opticalrecording medium 6.

[0038] With the optical pickup device 1 having the structure describedabove, the first laser beam L1 with a wavelength of 785 nm is emittedfrom the first laser beam light source 11 when recording data on a CD-Ras the optical recording medium 6. The first laser beam L1 is conductedto the common optical system Lo through the step-like diffractionelement 20, and converged as a light spot on the recording face of theCD-R by the objective lens 41 whereby the data is recorded.

[0039] In contrast, the second laser beam L2 with a wavelength of 650 nmis emitted from the second laser beam light source 12 when reproducingdata on a DVD as the optical recording medium 6. The second laser beamL2 is also conducted to the common optical system Lo through thestep-like diffraction element 20, and converged as a light spot on therecording face of the DVD by the objective lens 41; and returning lightof the second laser beam L2 reflected on the recording face of the DVDis condensed on the common light receiving element 13 through the beamsplitter 30. Data on the DVD is reproduced based on the signals detectedby the common light-receiving element 13.

[0040] (Step-Like Diffraction Element)

[0041] Description will be made with respect to the step-likediffraction element 20 that straightly advances the first laser beam L1with a long wavelength of 785 nm emitted from the two-wavelength lightsource 10 and deflects the second laser beam L2 with a short wavelengthof 650nm to match the optical axes of the two laser beams.

[0042]FIG. 2 is a cross-sectional view of a step-like diffractionelement to be used in the optical pickup device 1 shown in FIG. 1. FIG.3 is a graph showing relations between grating depths of the step-likediffraction element shown in FIG. 2 and the diffraction efficiency.

[0043] The step-like diffraction element 20 of the present embodiment isformed from a flat plate that is transparent to the wavelengths used, inwhich one of the faces defines a flat incident face 21 and the otherface defines an emission face 22 having a step-like grating surface. Thestep-like grating surface has periodically formed step-like gratings orstair-like gratings, each of which is composed of a plurality of stepfaces 23. In this embodiment, the number of step faces 23 is five.

[0044] A step difference d between adjacent ones of the step faces 23 ofthe step-like grating in the direction of an optical axis is set to havea measurement that generates a phase difference of 2π, in other words alight-path difference corresponding to one wavelength when the firstlaser beam L1 with a long wavelength of 785 nm transmits the grating.Accordingly, the step-like diffraction element 20 straightly advancesthe first laser beam L1 and deflects the second laser beam L2.

[0045] The number of steps of the step-like grating is determined to be“5” in the following manner. When the wavelength of the first laser beamon the long wavelength side is λ1, and the wavelength of the secondlaser beam on the short wavelength side is λ2, the number of steps isset to be an integer that is closest to α that satisfies the followingexpression (4).

λ2/λ1=α/(α+1)  (4)

[0046] In the present embodiment, the long wavelength λ1 is 785 nm, andthe short wavelength λ2 is 650 nm. With these values substituted in theexpression (4), the following expression (5) is obtained.

λ2/λ1=650/785=0.828 . . . =α/(α+1)  (5)

[0047] Therefore, the value α=4.813 . . . , and an integer that isclosest to the value α, which is “5”, is obtained.

[0048] The diffraction efficiency of the step-like diffraction element20 with the five step faces 23 may be given by the following expressions(6) and (7), when the zero-order diffraction efficiency is Fo and Morder diffraction efficiency is Fm.

Fo=1/5^(2{2) cos(2dπ)+2 cos(2dπ/2)+1}²  (6)

Fm=1/(πm)²[sin{π(2dm)}−sin{π(2d+3/5m)}+sin{−(d+3/5m)}−sin{π(d+1/5m)}+sin(1/5πm)]²  (7)

[0049] In these expressions, d is a height of one step between adjacentones of the step faces 23, and m is an order number of diffraction(which is an integer, but m≠0). Relations between the grating depths andthe diffraction efficiency with the long wavelength λ1=785 nm and theshort wavelength λ2=650 nm, which are derived from the expressions (6)and (7) are shown in a graph in FIG. 3. The grating depths are expressedby multiples of λ1 that represents the long wavelength of 785 nm.

[0050] As shown in the figure, when the total depth of the five stepfaces 23, in other words, the value of 4d, is changed from zero (0) to7λ1, no phase difference occurs for the long wavelength λ1=785 nm at 4λ1because the step difference d corresponds to one wavelength, and thezero-order diffraction efficiency becomes to be “1”, and the (−)first-order diffraction efficiency for the short wavelength λ2=650 nmbecomes to be 0.86, in other words, both of the diffraction efficienciesreach their respective maximum values.

[0051] Accordingly, the number of steps obtained by the expression (4),which is 5, maximizes the zero-order diffraction efficiency of the longwavelength λ1=785 nm and the (−) first-order diffraction efficiency ofthe short wavelength λ2=650 nm.

[0052] In contrast, when the step-like diffraction element 20 is formedin the number of steps that does not satisfy the expression (4), inother words, in the number of steps other than 5, the zero-orderdiffraction efficiency of the long wavelength and the (+) first-orderdiffraction efficiency or the (−) first-order diffraction efficiency ofthe short wavelength cannot be maximized.

[0053] In this embodiment, the collimator lens 40 generally satisfiesthe sine condition: and therefore, when an image height is h, and thefocal position of the long wavelength 785 nm of the collimator lens 40is F₇₆₅, a focal position ΔZ in the optical axis direction is given bythe following expression (8).

ΔZ≅F ₇₈₅−{square root}{square root over ( )}(F ₇₈₅ ² −h ²)  (8)

[0054] When a refractive index of the collimator lens 40 that ismanufactured with a material having a single uniform refractive indexagainst the laser beam with a short wavelength of 650 nm is N₆₅₀, and arefractive index of the collimator lens 40 against the laser beam with along wavelength of 785 nm is N₇₈₅, the amount of chromatic aberrationΔFc can be given by the following expression (9)

ΔFc=F ₇₈₅ −F ₇₈₅ N ₆₅₀ /N ₇₈₅  (9)

[0055] A focus deviation ΔF, which is caused by a positional difference(a height) in a vertical direction with respect to the optical axis ofthe laser beams emitted from the two different laser beam light sourcescan be given by adding the expressions (8) and (9) as indicated in anexpression (10) as follows.

ΔF=ΔZ+ΔFc≅F ₇₈₅−{square root}{square root over ( )}(F ₇₈₅ ² −h ²)+F ₇₈₅−F ₇₈₅ N ₆₅₀ /N ₇₈₅  (10)

[0056] The focus deviation ΔF can be corrected by forming the step-likestep surfaces 23 on the emitting face 22 of the step-like diffractionelement 20 in concentric curves, as indicated in FIG. 4.

[0057] In the optical pickup device 1 in accordance with the presentembodiment, the step-like diffraction element 20 is used to match theoptical axes of the laser beams L1 and L2 having different wavelengths,and the number of steps of the step faces 23 of the step-likediffraction element 20 is set such that the (−) first order diffractionefficiency of the laser beam L2 that receives the diffraction action ismaximized. Accordingly, the utilization efficiency of both of the lightsemitted from the first and second laser beam light sources 11 and 12.

[0058] (Second Embodiment)

[0059]FIG. 5 schematically shows a structure of an optical system of anoptical pickup device 1A in accordance with a second embodiment of thepresent invention. The optical pickup device 1A of the second embodimenthas a structure similar to that of the first embodiment, and accordinglydescriptions of elements common to both of the embodiments are omitted.

[0060] The optical pickup device 1A is equipped with a two-wavelengthlight source (light source unit) 10A that may includes as a commonpackage a first laser beam light source 11A that emits a first laserbeam L1A with a wavelength of 635 nm and a second laser beam lightsource 12A that emits a second laser beam L2A with a wavelength of 470nm. The optical pickup device 1A reproduces and records data on opticalrecording media 6 of different types, in which the first laser beam L1Awith a longer wavelength of 635 nm may be used to record data onDVD-RAMs and DVD-Rs, and the second laser beam L2A with a shorterwavelength of 470 nm may be used to reproduce data on super high densitydiscs.

[0061] A common optical system LoA includes a step-like diffractionelement 20A that straightly advances the first laser beam L1A emittedfrom the two-wavelength light source 10A and deflects the second laserbeam L2A to match optical axes of the both beams. The step-likediffraction element 20A will be described later in greater detail.

[0062] A first objective lens 41 a for the first laser beam L1A and asecond objective lens 41 b for the second laser beam L2A are disposed ina switchable manner to converge the laser beams L1A and L2A emitted fromthe collimator lens 40 on a recording surface 6 a of the optical medium6.

[0063] (Step-Like Diffraction Element)

[0064] Description will be made with respect to the step-likediffraction element 20A that straightly advances the first laser beamL1A emitted from the two-wavelength light source 10A and deflects thesecond laser beam L2A to match optical axes of the both beams.

[0065]FIG. 6 is a cross-sectional view of the step-like diffractionelement 20A to be used in the optical pickup device 1A shown in FIG. 5.FIG. 7 is a graph showing relations between grating depths of thestep-like diffraction element shown in FIG. 6 and the diffractionefficiency.

[0066] The step-like diffraction element 20A of the present embodimentis formed from a flat plate that is transparent to the wavelengths used,in which one of the faces defines a flat incident face 21A and the otherface defines an emission face 22A having a step-like surface. Thestep-like surface has periodically formed sets of steps or stairs, eachof which is composed of three step faces 23A.

[0067] A step difference d of the step faces 23A is set to have ameasurement that generates a phase difference of 2π, in other words alight-path difference corresponding to one wavelength when the firstlaser beam L1A with a long wavelength of 635 nm transmits the step-likediffraction element 20A. Accordingly, the step-like diffraction element20A straightly advances the first laser beam L1A and deflects the secondlaser beam L2A.

[0068] Also in this embodiment, when the wavelength of one of the firstand second laser beams L1 and L2 on the long wavelength side is λ1, andthe wavelength of the other laser beam on the short wavelength side isλ2, the number of step faces 23A, which is “3”, is obtained as aninteger that is closest to a that satisfies the expression (4). In thepresent embodiment, the long wavelength λ1 is 635 nm, and the shortwavelength λ2 is 470 nm, and therefore the expression (4) is modified tothe following expression (11).

λ2/λ1=470/635=α/(α+1)  (11)

[0069] Therefore, the value α=2.849 . . . , and an integer that isclosest to the value α, which is “3”, is obtained.

[0070] The diffraction efficiency of the step-like diffraction element20A with the three step faces 23A may be given by the followingexpressions (12) and (13), when the zero-order diffraction efficiency isFo and M order diffraction efficiency is Fm.

Fo=1/3²{2 cos (dπ)++1}²  (12)

Fm=1/(πm)²[sin{π(dm)}−sin{π(d+1/3m)}+sin{π(1/3m)}]²  (13)

[0071] In the expressions, d is a height of one step, and m is an ordernumber of diffraction (which is an integer but m≠0). Relations betweenthe grating depths and the diffraction efficiency with the longwavelength λ1=635 nm and the short wavelength λ2=470 nm, which arederived from the expressions (12) and (13) are shown in a graph in FIG.7. The grating depths are expressed by multiples of λ1 that representsthe long wavelength of 635 nm.

[0072] As shown in the figure, when the total depth of the three stepfaces, in other words, the value of 2d, is changed from zero (0) to 4λ1,no phase difference occurs for the long wavelength λ1=635 nm at 2λ1because the step difference d corresponds to one wavelength, and thezero-order diffraction efficiency becomes to be “1”, and the (−)first-order diffraction efficiency for the short wavelength λ2=470 nmbecomes to be 0.68, in other words, both of the diffraction efficienciesreach their respective maximum values.

[0073] Accordingly, the number of steps obtained by expression (4),which is 3, maximizes the zero-order diffraction efficiency of the longwavelength λ1=635 nm and the (−) first-order diffraction efficiency ofthe short wavelength λ2=470 nm.

[0074] Also, the collimator lens 40 generally satisfies the sinecondition, and therefore, when an image height is h, and the focalposition of the long wavelength 635 nm of the collimator lens 40 isF₆₃₅, a focal position ΔZ in the optical axis direction is given by thefollowing expression (14).

ΔZ≅F ₆₃₅−{square root}{square root over ( )}(F ₆₃₅ ² −h ²)  (14)

[0075] When a refractive index of the collimator lens 40 that ismanufactured with a material having a single uniform refractive indexagainst the laser beam with a wavelength of 470 nm is N₄₇₀, and arefractive index thereof against the laser beam with a wavelength of 635nm is N₆₃₅, a color aberration ΔFc can be given by the followingexpression (15).

ΔFc=F ₆₃₅ −F ₆₃₅ N ₄₇₀ /N ₆₃₅  (15)

[0076] A focus deviation ΔF, which is caused by a positional difference(a height) in a vertical direction with respect to the optical axis ofthe laser beams emitted from the two different laser beam light sourcescan be given by adding the expressions (14) and (15) as indicated in thefollowing expression (16).

ΔF=ΔZ+ΔFc≅F ₆₃₅−{square root}{square root over ( )}(F ₆₃₅ ² −h ²)+F₆₃₅−F ₆₃₅ N ₄₇₀ /N ₆₃₅  (16)

[0077] This focus deviation ΔF can be corrected similarly as describedin the first embodiment by forming the step-like step surfaces 23A onthe emitting face 22A of the step-like diffraction element 20A inconcentric curves, as indicated in FIG. 4.

[0078] [Another Example of First Embodiment]

[0079] The step-like diffraction element 20 in the optical pickup device1 of the first embodiment straightly advances the first laser beam L1with a long wavelength of 780 nm, and deflects the second laser beam L2with a short wavelength of 650 nm to match the optical axes of the laserbeams. However, the step-like diffraction element 20 can be structuredsuch that the laser beam L1 with a long wavelength may be deflected, andthe laser beam L2 with a short wavelength may travel straight paththrough the step-like diffraction element 20 to mach the optical axes ofthe laser beams. The following description will be made with thewavelength λ1 of the first laser beam L1 on the long wavelength sidebeing 785 nm and the wavelength λ2 of the second laser beam L2 on theshort wavelength side being 635 nm.

[0080]FIG. 8 is a cross-sectional view of a step-like diffractionelement in this embodiment. FIG. 9 is a graph showing relations betweengrating depths of the step-like diffraction element shown in FIG. 8 andthe diffraction efficiency.

[0081] The step-like diffraction element 20B in this embodiment deflectsthe first laser beam L1 with a wavelength of 780 nm, and straightlyadvances the second laser beam L2 with a wavelength of 635 nm to matchthe optical axes of the laser beams. The step-like diffraction element20B is formed from a flat plate that is transparent to the wavelengthsused, in which one of the faces defines a flat incident face 21B and theother face defines an emission face 22B having a step-like surface. Thestep-like surface 22B has periodically formed sets of steps or stairs,each of which is composed of five step faces 23B.

[0082] The five step faces 23B are set such that a step difference dthereof is set to have a measurement that generates a phase differenceof 2π, in other words a light-path difference corresponding to onewavelength when the second laser beam L2 with a short wavelength of 635nm transmits the step-like diffraction element 20B. Accordingly, thestep-like diffraction element 20B deflects the first laser beam L1 andstraightly advances the second laser beam L2, to thereby match theoptical axes of the laser beams.

[0083] Unlike the first and second embodiments, the number of step faces23B is decided in a different manner in this embodiment as follows. Whenthe wavelength on the long wavelength side among the first and secondlaser beams L1 and L2 is λ1, and the wavelength on the short wavelengthside is λ2, the number of steps is set to a value in which one “1” isadded to a value α that satisfies the expression (4). When the longwavelength λ1 is 785 nm, and the short wavelength λ2 is 635 nm, theexpression (4) can be represented by the following expression (17).

λ2/λ1=635/785=α/(α+1)  (17)

[0084] Therefore, since the value α=4.235 . . . is derived from theexpression (17), an integer that is closest to the value α is “4”, andtherefore the number of steps, which is 5(=α+1), is obtained.

[0085] The diffraction efficiency of the step-like diffraction element20B with the five steps thus configured may be given by the expressions(6) and (7) similarly as given in the first embodiment, with thezero-order diffraction efficiency being Fo and M order diffractionefficiency being Fm. Relations between the grating depths and thediffraction efficiency with the long wavelength λ1=785 nm and the shortwavelength λ2=635 nm, which are derived from the expressions (6) and (7)are shown in a graph in FIG. 9. The grating depths are expressed bymultiples of λ2 that represents the short wavelength of 635 nm.

[0086] As shown in the figure, when the total depth of the five steps ofthe step-like diffraction element, in other words, the value of 4d, ischanged from zero (0) to 7λ2, no phase difference occurs for the shortwavelength λ2=635 nm at 4λ2 because the step difference d corresponds toone wavelength, and the zero-order diffraction efficiency becomes to beone “1”, and the (+) first-order diffraction efficiency for the longwavelength λ1=785 nm becomes to be 0.86, in other words, both of thediffraction efficiencies reach their respective maximum values.Accordingly, the number of steps obtained by expression (4), which is 5,maximizes the (+) first-order diffraction efficiency of the longwavelength λ1=785 nm and the zero-order diffraction efficiency of theshort wavelength λ2=635 nm.

[0087] When the step-like diffraction element 20B is formed to have thenumber of steps that does not satisfy expression (4), in other words,the number of steps other than 5, the maximum value of the (+)first-order diffraction efficiency of the long wavelength does notcoincide with the maximum value of the zero-order diffraction efficiencyof the short wavelength, even when one wavelength of the shortwavelength is made equal to the height of one step.

[0088] With the image height characteristic of the collimator lens 40,when an image height is h, and the focal position of the shortwavelength of 635 nm of the collimator lens 40 is F₆₃₅, a focal positionΔZ in the optical axis direction is given by the following expression(18).

ΔZ=F ₆₃₅−(F ₆₃₅ ² −h ²)  (18)

[0089] When a refractive index of the collimator lens 40 that ismanufactured with a material having a single uniform refractive indexagainst the laser beam with a wavelength of 635 nm is N₆₃₅, and arefractive index thereof against the laser beam with a wavelength of 785nm is N₇₈₅, the amount of chromatic aberration ΔFc can be given by thefollowing expression (19)

ΔFc=F ₆₃₅ −F ₆₃₅ N ₇₈₅ /N ₆₃₅  (19)

[0090] A focus deviation ΔF, which is caused by a positional difference(a height) in a vertical direction with respect to the optical axis ofthe laser beams emitted from the two different laser beam light sourcescan be given by adding the expressions (18) and (19) as indicated in thefollowing expression (20).

ΔF=ΔZ+ΔFc≅F₆₃₅−{square root}{square root over ( )}(F ₆₃₅ ² −h ²)+F ₆₃₅−F ₆₃₅ N ₇₈₅ /N ₆₃₅  (20)

[0091] This focus deviation ΔF can be corrected by forming the step-likestep surfaces 23B on the emitting face 22B of the step-like diffractionelement 20B in concentric curves, as indicated in FIG. 4, in a similarmanner as done in the first embodiment.

[0092] With the optical pickup device equipped with the step-likediffraction element 20B, in any of the occasions when the first laserbeam L1 with a long wavelength is used to reproduce data on a CD-R, andwhen the second laser beam L1 with a short wavelength is used toreproduce and record data on a CVD, DVD-RAM or DVD-R, the utilizationefficiency of the laser beams can be raised to their maximum values.

[0093] (Another Example of Second Embodiment)

[0094] Similarly, the step-like diffraction element 20A in the opticalpickup device 1A of the second embodiment straightly advances the firstlaser beam L1 with a long wavelength of 635 nm, and deflects the secondlaser beam L2 with a short wavelength of 470 nm to match the opticalaxes of the laser beams. However, the step-like diffraction element 20Acan be structured such that the laser beam L1 with a long wavelength maybe deflected, and the laser beam L2 with a short wavelength may bestraightly advanced through the step-like diffraction element 20A tomach the optical axes of the laser beams.

[0095]FIG. 10 is a cross-sectional view of a step-like diffractionelement 20C of another example to be used in the optical pickup device1A shown in FIG. 5.

[0096] The step-like diffraction element 20C in this embodiment deflectsthe first laser beam L1A with a wavelength of 635 nm, and straightlyadvances the second laser beam L2A with a wavelength of 470 nm to matchthe optical axes of the laser beams. The step-like diffraction element20C is formed from a flat plate that is transparent to lights that areused, in which one of the faces defines a flat incident face 21C and theother face defines an emission face 22C having a step-like surface. Thestep-like surface 22C has periodically formed sets of steps or stairs,each of which is composed of four step faces 23C.

[0097] The four step faces 23C are set such that a step difference dthereof is set to generate a phase difference of 2π, in other words alight-path difference corresponding to one wavelength when the secondlaser beam L2A with a wavelength of 470 nm transmits the step-likediffraction element 20C. Accordingly, the step-like diffraction element20C deflects the first laser beam L1A and straightly advances the secondlaser beam L2, to thereby match the optical axes of the laser beams.

[0098] When the wavelength on the long wavelength side among the firstand second laser beams L1A and L2A is λ1, and the wavelength on theshort wavelength side is λ2, the number of steps is set to be “4”.Namely, since the value α=2.857 . . . is derived from the expression(4), an integer that is closest to the value α is “3”, and therefore thenumber of steps, which is 4 (=α+1), is obtained.

[0099] With the optical pickup device equipped with the four-stepstep-like diffraction element 20C, the (+) first-order diffractionefficiency of the first laser beam L1A with a wavelength of 635 nm is0.81, and the zero-order diffraction efficiency of the laser beam L2Awith a wavelength of 470 nm is “1”.

[0100] (Other Embodiments)

[0101] In the examples described above, a step-like diffraction elementis disposed in an optical path that starts from a first laser beam lightsource and a second laser beam light source, and reaches an objectivelens in a common optical system, in other words, a forward path in theoptical system. Instead, the step-like diffraction element may bedisposed in a returning path of the optical system that reaches from anobjective lens to a light receiving element, for example, between a beamsplitter in the common optical system and the common light-receivingelement.

[0102] In this case, returning lights of the first and second laserbeams, which are conducted through the common optical system with theiroptical axes being deviated, can be adjusted to match their optical axesby the step-like diffraction element and received by the common lightreceiving element.

[0103] As describe above, a two-wavelength optical pickup device inaccordance with the present invention uses a step-like diffractionelement as a phase diffraction element that is used to match opticalaxes of lights with different wavelengths, and the number of steps inthe step-like diffraction element is set to maximize the utilityefficiency of both of the laser beams. Accordingly, the lights havingdifferent wavelengths emitted from different light sources can beconducted to a common optical system through the step-like diffractionelement in a state with a few light loss amount. In this manner, inaccordance with the present invention, the utility efficiency of emittedlights can be increased, and therefore the power output of the lightsources can be reduced, and a lower power consumption of the device canbe realized.

[0104] While the description above refers to particular embodiments ofthe present invention, it will be understood that many modifications maybe made without departing from the spirit thereof The accompanyingclaims are intended to cover such modifications as would fall within thetrue scope and spirit of the present invention.

[0105] The presently disclosed embodiments are therefore to beconsidered in all respects as illustrative and not restrictive, thescope of the invention being indicated by the appended claims, ratherthan the foregoing description, and all changes which come within themeaning and range of equivalency of the claims are therefore intended tobe embraced therein.

What is claimed is:
 1. An optical pickup device comprising: a firstlight source that emits first light; a second light source that emitssecond light with a wavelength different from a wavelength of the firstlight; and a step-like diffraction element that deflects one of thefirst light and the second light to match optical axes of the firstlight and the second light, the step-like diffraction element defining alight incident face and a light emitting face, one of the incident faceand the emitting face having multiple sets of step-like grating faces,wherein a step difference between adjacent ones of each one of themultiple sets of the step-like grating faces has a measurement thatgenerates a phase difference corresponding to one wavelength of one ofthe first light and the second light, and the number of steps of each ofthe multiple sets of step-like grating faces is set at a value thatmaximizes one of a (+) first order diffraction efficiency and a (−)first order diffraction efficiency for the other of the first light andthe second light.
 2. An optical pickup device according to claim 1,wherein the step-like diffraction element straightly advances one of thefirst light and the second light, and deflects the other of the firstlight and the second light to thereby match the optical axes of thefirst light and the second light.
 3. An optical pickup device accordingto claim 2, wherein the step-like grating faces of the step-likediffraction element are formed in concentric circular curves when viewedfrom the emitting face of the step-like diffraction element.
 4. Anoptical pickup device comprising: a first light source that emits firstlight; a second light source that emits second light having a wavelengthdifferent from a wavelength of the first light; and a step-likediffraction element that deflects one of the first light and the secondlight to match optical axes of the first light and the second light, thestep-like diffraction element defining a light incident face and a lightemitting face, one of the light incident face and the light emittingface having multiple sets of step-like grating faces, wherein, when awavelength λ1 of the first light is longer than a wavelength λ2 of thesecond light, a step difference between adjacent ones of the step-likegrating faces has a measurement that generates a phase differencecorresponding to one wavelength for the first light, and the number ofsteps of each of the multiple sets of step-like grating faces is set atan integer that is closest to a value α that satisfies an expression ofλ2/λ1=α/(α+1).
 5. An optical pickup device according to claim 4,wherein, when the wavelength λ1 of the first light is about 785 nm, andthe wavelength λ2 of the second light is about 650 nm, the number ofsteps of each of the multiple sets of step-like grating faces is five.6. An optical pickup device according to claim 4, wherein, when thewavelength λ1 of the first light is about 635 nm, and the wavelength λ2of the second light is about 470 nm, the number of steps of each of themultiple sets of step-like grating faces is three.
 7. An optical pickupdevice comprising: a first light source that emits first light; a secondlight source that emits second light having a wavelength different froma wavelength of the first light; and a step-like diffraction elementthat deflects one of the first light and the second light to matchoptical axes of the first light and the second light, the step-likediffraction element defining a light incident face and a light emittingface, one of the light incident face and the light emitting face havingmultiple sets of step-like grating faces, wherein, when a wavelength λ1of the first light is longer than a wavelength λ2 of the second light, astep difference between adjacent ones of the step-like grating faces ofeach of the multiple sets of step-like grating faces has a measurementthat generates a phase difference corresponding to one wavelength forthe second light, and the number of steps of each of the multiple setsof step-like grating faces is set at a value in which one is added to aninteger that is closest to a value α that satisfies an expression ofλ2/λ1=α(α+1).
 8. An optical pickup device according to claim 7, wherein,when the wavelength λ1 of the first light is about 785 nm, and thewavelength λ2 of the second light is about 635 nm, the number of stepsof each of the multiple sets of step-like grating faces is five.
 9. Anoptical pickup device according to claim 7, wherein, when the wavelengthλ1 of the first light is about 635 nm, and the wavelength λ2 of thesecond light is about 470 nm, the number of steps of each of themultiple sets of step-like grating faces is four.
 10. A method fordesigning an optical pickup device comprising a first light source thatemits first light, a second light source that emits second light havinga wavelength different from a wavelength of the first light, and astep-like diffraction element that deflects one of the first light andthe second light to match optical axes of the first light and the secondlight, the step-like diffraction element defining a light incident faceand a light emitting face, one of the light incident face and the lightemitting face having multiple sets of step-like grating faces, themethod comprising the steps of: when a wavelength λ1 of the first lightis longer than a wavelength λ2 of the second light, setting a stepdifference between adjacent ones of the step-like grating faces to havea measurement that generates a phase difference corresponding to onewavelength for the first light; and setting the number of steps of eachof the multiple sets of step-like grating faces to be at an integer thatis closest to a value α that satisfies an expression of λ2/λ1=α/(α+1).11. A method for designing an optical pickup device comprising a firstlight source that emits first light, a second light source that emitssecond light having a wavelength different from a wavelength of thefirst light, and a step-like diffraction element that deflects one ofthe first light and the second light to match optical axes of the firstlight and the second light, the step-like diffraction element defining alight incident face and a light emitting face, one of the light incidentface and the light emitting face having multiple sets of step-likegrating faces, the method comprising the steps of: when a wavelength λ1of the first light is longer than a wavelength λ2 of the second light,setting a step difference between adjacent ones of the step-like gratingfaces of each of the multiple sets of step-like grating faces to have ameasurement that generates a phase difference corresponding to onewavelength for the second light; and setting the number of steps of eachof the multiple sets of step-like grating faces to be at a value inwhich one is added to an integer that is closest to a value α thatsatisfies an expression of λ2/λ1=α/(α+1).